1. Field of the Invention
The present invention relates to an intake air amount control apparatus for a vehicle engine provided with a variable valve mechanism which varies a lift amount of an intake valve and a brake booster for assuring a master vacuum negative pressure by using a negative pressure of the engine, and a method thereof.
2. Description of the Related Art
In a valve characteristic control apparatus disclosed in Japanese Unexamined Patent Publication No. 2001-254637, it is diagnosed whether or not a variable valve lift mechanism is failed, based on a target lift amount and an actual lift amount of an engine valve.
Further, in the above valve characteristic control apparatus, when it is detected that the variable valve lift mechanism is failed, there are performed the processing of: increasing a fuel supply amount; prohibiting a leaning control; extending an operation region where the fuel supply is cut off; and correcting an engine rotation speed for resuming the fuel supply from the fuel supply cut off condition.
However, if an intake valve is fixed at a low lift condition due to the failure of the variable valve lift mechanism, an opening area of the intake valve becomes small, so that an amount of air sucked into a cylinder is significantly restricted.
Therefore, in the condition where the intake valve is fixed at the low lift condition, in order to increase the engine rotation speed in a tow load area to enhance the stability, an opening of an intake throttle valve, such as a throttle valve or the like, needs to be made larger, to achieve an increase of a cylinder intake air amount.
However, if the opening of the intake throttle valve is made larger, since an intake negative pressure of the engine approaches the atmospheric pressure, a brake booster for assuring a master vacuum negative pressure by using the negative pressure of the engine is not operated normally, resulting in the reduction of braking force.